A built-up camshaft is understood to mean a camshaft that is produced not in one piece in a primary forming or shaping process, but, rather, by connecting multiple prefabricated parts together. Recent built-up camshafts have, in addition to cams, further functional elements such as end pieces, a chain wheel, belt pulleys, sensor wheels, pump drives, roller bearings, slide bearings, or the like, that must be fastened to a support tube using a suitable joining technology, for which very high angular accuracies must also be maintained.
In practice, making built-up camshafts by hydroforming has proven successful, in that in one process step multiple cams and/or other functional elements are simultaneously fastened to a support tube with a press fit. In hydroforming, the cams and the other functional elements that may be provided are initially pushed with play onto the support tube with their hubs and positioned, the support tube subsequently being acted on by a high internal pressure, as the result of which the support tube is expanded elastically and plastically and fitted against the inner surface of the hubs. The cams and other functional elements that may be provided are also expanded to a certain extent, although this expansion preferably occurs only elastically. After the action of pressure is discontinued, the support tube as well as the elements situated thereon spring back elastically, the materials of the joining partners being selected in such a way that the cams and other functional elements are held on the support tube with a press fit via elastic residual stress.
Hydroforming is usually carried out in a mold that limits radial expansion due to the action of pressure. According to one preferred embodiment of the hydroforming process, when the support tube is acted on by a pressurized fluid over its entire length, an adequate seal must be ensured. In addition, in hydroforming it is the aim to allow reliable transmission of high torques with the lowest possible material use and with a simple construction.
When hydroforming is carried out in a mold, a pressure source is usually connected at one end of the support tube, while the end of the support tube opposite the pressure source must be sealed pressure-tight, for which purpose end pieces that are mounted like a cap may be used. Another variant is the option for pressure medium entering through an axial borehole in the end piece. The tube is sealed pressure-tight on the opposite end. A reliable seal is necessary to avoid the uncontrolled escape of pressurized fluid, and thus, failure of the joining process.
A method of making a built-up camshaft having the features described above is known from DE 36 16 901 [U.S. Pat. No. 4,660,269]. The production of the tubular structural element in the form of a built-up camshaft is carried out in a closed mold, a source for the pressurized fluid being connected to one end of the support tube, and end an end piece being pushed onto the other end of the support tube during the hydroforming process. Various measures are proposed for achieving a seal. Placing a sealing ring in a radial groove or at the base of the cavity in the end piece is complicated, and leaks of the polymeric sealing material cannot be ruled out on account of the very high pressures of typically 2000 to 5000 bar that act during hydroforming. Alternatively, it is proposed to provide the cavity as a blind hole with a frustoconical bevel at its base against that the support tube is pressed. Only linear contact is gained around the periphery, which cannot ensure a reliable seal in the event of dimensional deviations, out-of-round eccentricities, and localized damage. In addition, producing such frustoconical surfaces at the base of the cavity is complicated.